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Journal of Rock Mechanics and Geotechnical Engineering. 2012, 4 (3): 215227
Design of rock support system under rockburst condition
Peter K. Kaiser1, Ming Cai
2
1Centre for Excellence in Mining Innovation,Sudbury,Ontario,Canada
2Bharti School of Engineering,Laurentian University,Sudbury,Ontario,Canada
Received 14 April 2012;received in revised form 12 June 2012; accepted 9 July 2012
Abstract: As mining and civil tunneling progresses to depth, excavation-induced seismicity and rockburst problems
increase and cannot be prevented. As an important line of defense, ground control measures and burst-resistant rock support
are used to prevent or minimize damage to excavations and thus to enhance workplace safety. Rock support in burst-prone
ground differs from conventional rock support where controlling gravity-induced rockfalls and managing shallow zones ofloose rock are the main target. Rock support in burst-prone ground needs to resist dynamic loads and large rock dilation due
to violent rock failure. After reviewing the rockburst phenomenon, types of rockbursts, damage mechanisms, and rockburst
support design principles and acceptability criteria, this paper describes that the support selection process in burst-prone
ground is iterative, requiring design verification and modification based on field observations. An interactive design tool for
conducting rockburst support design in underground tunnels is introduced to facilitate cost-effective design.
Key words:rockburst; rockburst damage; rock support; design
1 Introduction
As the depth of mining and civil underground
construction increases, stress-induced rock fracturing
is inevitable and when stored energy is suddenly
released, rocks fail violently, leading to seismic
events and rockbursts. A rockburst is defined as
damage to an excavation that occurs in a sudden or
violent manner and is associated with a seismic event
(Hedley, 1992; Kaiser et al., 1996). Many hard rock
mines in Canada, China, Chile, South Africa,
Australia, Sweden, and other countries, and some
deep civil tunnels in Switzerland, China, and Peruhave experienced rockbursts to various degrees. Two
recent civil projects that experienced severe
rockburst damage are the Jinping II hydropower
intake tunnels in China and the Olmos Trans-Andean
tunnel in Peru.
Considerable research effort, at an international
scale (e.g. Australia, Canada, South Africa, China),
has been devoted to the understanding of the
rockburst phenomenon. Micro-seismic monitoring
Doi: 10.3724/SP.J.1235.2012.00215Corresponding author. Tel: +1-705-675-1151;
E-mail: [email protected]
Supported by NSERC (Canada), LKAB (Sweden), VALE (Canada),
CEMI (Canada), XSTRATA NICKEL (Canada), MIRARCO (Canada)
systems are in operation at most burst-prone mines
and tunnel construction sites around the world. From
the waveform records, the time, location, radiated
energy, seismic moment and other source parameters
of a seismic event can be obtained. Monitoring of
seismic events in mines or along tunnels therefore is
a very useful tool for outlining potentially hazardous
ground conditions and assisting construction
management in effective re-entry decision-making.
Advanced three-dimensional (3D) numerical
modeling and visualization can identify potentially
hazardous areas and assist in planning and design
underground structures.
Rockburst risk can often be reduced by selectingappropriate mining or excavation methods and
sequences, and by strategically placing developments
and other infrastructure. However, due to
uncertainties in rock mass properties and boundary
conditions (e.g. in-situ stress, fault zone distribution),
engineering design will have to rely on ground
control measures with burst-resistant rock support as
an important line of defense to ensure workplace
safety. For this reason, it is imperative to design
proper burst-resistant support systems when mining
and tunneling at depth. No excavation in burst-proneground should be advanced without the installation
of burst-resistant support systems (Stacey, 2011).
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The design of rock support in burst-prone grounds
differs from conventional rock support where
controlling gravity-induced rockfalls and managing
shallow zones of loose rock are the main target. Rock
support in burst-prone ground needs to resist
dynamic loads and large deformations due to rock
dilation, called bulking, during the violent failure of
rock. The term bulking is used to describe volume
increases of the rock mass near an excavation due to
geometric non-fit during the transition from
competent to fractured and then to broken rocks.
Near excavations, bulking is unidirectional toward
the excavation (perpendicular to the wall), a function
of the applied tangential strain, and highly dependent
on the confining stress. For this purpose of rock
support in burst-prone ground, the designers must
understand the rockburst damage mechanisms, assess
the rock support demands, and be able to select the
right support products to fulfill several support
functions. Furthermore, the 3D complex geological
and geometrical conditions as well as the uncertainty
or variability of design input parameters complicate
the design. Hence, rock support design becomes an
interactive and iterative process of selecting proper
support elements to form a rock support system
which has enough capacity to meet the expected
demands.
Because of these complexities, it becomes quickly
evident that such a design process cannot be carried
out for all underground excavations in a consistent
manner if the design is conducted manually.
Tremendous time and effort would be required to
manually conduct such design work and costly
mistakes could be made if the design engineers do
not pay attention to details. Hence, a design guideline
which explains the principles and methodologies as
well as rock support system capacities is required for
design professionals. Furthermore, a rockburst
support design tool which helps to streamline the
design process and integrate past and current
knowledge is needed for the mining and civil
construction industries.
In response to industrys needs, an R&D project is
currently on-going at Laurentian University in
Canada to produce a concise design guide and to
develop an interactive design tool for rock support
design in burst-prone grounds. In this paper, after
reviewing the rockburst phenomenon, types of
rockbursts, damage mechanisms, rockburst support
design principles and design acceptability criteria,
the design tool which can be used to facilitate a
systematic and consistent rock support design in
burst-prone grounds is introduced.
2 Rockbursting and rockburst
damage
2.1 Rockburst phenomenon
Rockburst is a 20th century phenomenon as the
first recorded incident occurred in the early 1900s in
the gold mines in the Witwatersrand, South Africa
(Blake and Hedley, 2003). Rockbursting is the result
of sudden and violent failure of rocks. There is a
clear linkage between rockburst activities and mining
depth. As mining migrates to deeper ground, in-situ
stress becomes high relative to the rock strength and
the likelihood of rockburst drastically increases.
Rockbursts are mostly associated with hard rocks andgeological structures such as faults and dykes and in
mining are often related to high extraction ratios and
associated with mining methods causing unfavorable
stress conditions.
2.2
Types of rockbursts
Ortlepp and Stacey (1994) and Ortlepp (1997)
classified rockbursts into five types (strainburst,
buckling, face crush/pillar burst, shear rupture,
fault-slip burst). In a broad sense, buckling type
rockbursts can be grouped into strainbursts, and shear
rupture type rockbursts can be considered as
fault-slip rockbursts. For brevity of discussion, we
consider here three rockburst types, i.e. strainburst,
pillar burst, and fault-slip burst. Rockbursts are either
mining-induced by energy release causing damage at
the source (e.g. strainburst without significant
dynamic stress increase from a remote seismic event)
or dynamically-induced rockbursts with damage
caused by energy transfer or significant dynamic
stress increase from a remote seismic event (e.g.
strainburst with dynamic stress increase caused by aremote seismic event).
Rock mass failure occurs when the excavation-
induced stress exceeds the peak strength of the rock
mass. In many deep underground excavations,
strainbursts are the most common rockburst type;
they can be mining-induced due to static stress
change caused by nearby mining or dynamically-
induced due to dynamic stress increase caused by a
remote seismic event (called dynamically-induced
strainbursts). An example of strainburst damage is
shown in Fig. 1.
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Fig. 1 Example of strainburst damage to a supported
excavation.
Two conditions must be met for a strainburst to
occur. First, the tangential stress (the maximum
principal stress) must be able to build up in the
immediate skin of the excavation. Second, the rockmass surrounding the fracturing rock must create a
relatively soft loading environment such that the
rock fails locally in an unstable, violent manner. The
energy released by a strainburst comes from the
stored elastic strain energy in the failing rock and the
surrounding rock mass (not from the seismic source).
During tunnel and shaft construction, strainbursts
normally occur within three times the diameter from
the advancing face. Such strainbursts can also occur
right at the tunnel face and in the floor. In mining,
stress changes in the drifts (horizontal tunnels in amine) may occur after development due to stopping
activities; consequently, mining-induced strainbursts
can happen during the production stage. Delayed
strainbursts occur in situations where the maximum
principal stress remains constant but the rock
strength degrades over time, or the rock strength
reduces due to loss of confinement.
Due to a potentially unstable equilibrium situation
near an excavation, strainbursts may be triggered by
a small dynamic disturbance, a production blast, a
remote pillar burst or fault slip event. For suchdynamically-triggered strainbursts, little or none of
the released energy stems from the triggering event.
Instead, the stored strain energy at the bursting
location and the surrounding rock constitutes most of
the release energy.
Pillar burst, as the name implies, is defined as a
violent failure in the pillar core or the complete
collapse of a pillar. Pillar bursts often occur in deep
mines when the extraction ratio is high at a later
stage of mining. The volume of failed rock and the
affected surrounding rock mass is usually larger thanthat involved in a strainburst and hence the released
seismic energy is much greater.
Similar to strainburst, pillar burst can be classified
into mining-induced pillar burst and dynamically-
induced pillar burst. A mining-induced pillar burst is
caused by static stress increase from increased room
span or nearby stope extraction. The seismic source
is in the confined core of the pillar, and rockburst
damage and seismic source are co-located. On the
other hand, a dynamically-induced pillar burst is
caused by dynamic stress increase from a remote
seismic event. In this case, the rockburst damage and
the seismic source (i.e. fault-slip event) are not
co-located. An example of pillar burst is shown in
Fig. 2.
Fig. 2An example of pillar burst (Hedley, 1992).
A fault-slip burst is caused by the dynamic
slippage along a pre-existing fault or along a newly
generated shear rupture. A critically stressed fault,
with shear stresses exceeding the shear strength, can
slip when the degree of freedom is changed as it is
intersected by a mine opening. Alternatively, it may
slip when the shear strength is reduced due to a drop
in clamping stress or water infiltration into the fault.
Finally, it may slip when the mining-induced shear
stress is increased and exceeds the strength of the
fault, which is a function of the normal stress, the
coefficient of friction of the fault surface, its
waviness or dilation characteristics, and, in the caseof fracture propagation, the strength of the rock mass.
Similar to pillar burst, fault-slip rockbursts occur
in deep mines when the extraction ratio is high and
large closures are allowed to persist over large
mining volumes. The most plausible cause of
fault-slip along a pre-existing fault is the reduction of
normal stress acting on the fault as a result of nearby
mining, although an increase in shear stress or a
combination of normal stress decrease and shear
stress increase can similarly cause a fault to slip. This
type of rockburst may release a large amount ofseismic energy, coming from the instantaneous
relaxation of elastic strain stored in a large volume of
Pillar
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highly stressed rock surrounding the slip or rupture
area. They may create sufficiently high ground
vibrations or ground motions that can cause damage
to excavations (dynamically-induced strainbursts),
cause shake down of loose or insufficiently
supported rock, and/or trigger strainburst and pillar
burst at relatively remote locations (hundreds of
meters from the seismic source).
Shear rupture type rockbursts have been observed
in some mines, particularly in South African mines
(Ortlepp, 1997, 2000). Large rockbursts, with Richter
magnitude exceeding 3.5, can result from violent
propagation of shear fracture through intact rocks.
Ortlepp (1997) strongly advocated shear rupture as
one of the most important source mechanisms for
major rockbursts. There is however a possibility that
his bias is in part influenced by the relatively soft
mining system stiffness encountered in tabular ore
bodies in South Africa.
2.3 Rockburst damage mechanism
Understanding the rockburst source mechanism is
critical to deriving strategies to eliminate and
mitigate rockburst hazard, and a thorough
understanding of the rockburst damage mechanism is
needed to work out tactics to implement rockburst
support.
Kaiser et al. (1996) classified rockburst damage
into three types, i.e. rock bulking due to fracturing,
rock ejection due to seismic energy transfer, and
rockfall induced by seismic shaking (Fig. 3). Rock
bulking due to rock fracturing can be caused by both
a remote seismic event and the bursting event itself.
Brittle rock fracturing occurs as a result of crack and
fracture initiation, propagation, and coalescence
(Kaiser et al., 2000; Cai et al., 2004). This leads to
the generation of new fracture surfaces in a
Fig. 3 Rockburst damage mechanism, damage severity, and
required support functions (modified from Kaiser et al.
(1996)).
previously intact or less fractured media and, as a
consequence, this rock mass disintegration leads to
rock mass bulking. This bulking process is in large
part a result of geometric block incompatibilities and
thus is much larger than dilation during plastic rock
mass yield. Most importantly, it is directional,
perpendicular to the excavation wall. During bulking,
the broken rock volume increases as it is fractured
and fragmented.
Rock ejection can be caused by a strainburst event,
a pillar burst event, or by a remote seismic event
through dynamic moment transfer. Ejected rock may
travel at velocities in excess of 3 m/s; velocities up to
10 m/s were estimated by Ortlepp and Stacey (1994).
The upper end of this ejection velocity range cannot
be explained by the moment transfer damage
mechanism alone. When rock suddenly fractures,
part of the stored strain energy in the surrounding
rocks can be transferred to blocks in the form of
kinetic energy, causing rock ejection. With high
strain energy stored in the rock near the excavation,
the stress wave from a remote seismic event may add
a dynamic stress disturbance and cause a strainburst
(bring the bucket to overflow). In this case, the
ejection velocity is not directly related to the
momentum from the seismic source but more closely
related to the energy stored in the near-wall rock and
how this stored energy is released.
Seismically-induced rockfalls, as the name
suggests, are caused by the (low frequency) shaking
of ground due to a large remote seismic event,
perhaps induced by a pillar burst or a fault-slip
rockburst. It occurs when an incoming seismic wave
accelerates a volume of rock that was previously
stable under static loading conditions, causing forces
that overcome the capacity of the support system.
Note that it is also possible that the first incoming
seismic wave may fracture a volume of rock, and
subsequent vibration induced by the seismic wavesaccelerates the fractured rocks, causing falls of
ground. Seismically-induced rockfalls occur
frequently at intersections where the span is large and
roof rock confinement is low.
2.4 Factors influencing rockburst damage
There are many factors that influence rockburst
damage and the severity of the damage (Hedley,
1992; Kaiser et al., 1996; Durrheim et al., 1998; Heal
et al., 2006; Cai and Champaigne, 2009). Fig. 4
summarizes the main factors and groups them into
four categories, i.e. seismic event, geology, geotechnical,and mining. Factors in the first two groups (seismic
event and geology) determine the intensity of
Reinforce
Hold
Retain
Required
support functionsDamage severity
0.25 m
0.75 m
1.5 m
Moderate
Minor
MajorSeismically-induced
rockfall
Rock ejection due to
strainburst or seismic
energy transfer
Rock bulking
due to fracturing
Damage mechanism
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Fig. 4Main factors influencing rockburst damage.
dynamic load at the damage locations, and the factors
in the last two groups (geotechnical and mining)
determine site response due to seismic impulses.
Rockburst damage is therefore governed by a
combination of these factors.
When the size of an opening is large or when
multiple openings are created close to each other, the
chance of having a rockburst is greatly increased due
to a reduction in loading system stiffness. Hence,
excavation or yield zone geometry can also influence
the rockburst propensity.
When geological weaknesses, such as faults or
shear zones, or stress raisers, such as dykes, are
nearby, the released energy may often be larger
because these geological structures tend to create
unfavorable stress and loading system conditions, e.g.
by involving large rock volumes in the deformation
and failure process.
Rockburst damage severity is often classified bythe depth of failure or the volume of rock failed and
the degree of damage to the installed rock support
system. A three-class (minor, moderate, and major)
classification can be found in Fig. 3.
It is interesting to note from Fig. 4 that many
factors, such as mining sequence, excavation span,
and installed rock support system are in the mining
activity category. These factors are created by mining
operations, and hence working on these factors
provides manageable means to reduce and controlrockburst damage potential. There are many methods
to achieve this goal, such as changing the mining
method, altering the mining sequence, changing drift
locations, etc. This is where having a good
underground construction strategy will pay off
quickly. It should be pointed out that selecting a good
construction strategy is necessary but not sufficient
to create a safe work environment; a rockburst
support plan needs to be implemented in parallel.
The importance of having effective rock support
systems in bursting ground has been demonstrated by
numerous case histories. In the following sections,
we discuss rockburst support design principles and
methodologies and present a tool for designing rock
support systems for highly stressed, burst-prone
tunnels.
3 Rockburst support designprinciples and methodologies
3.1 Rock support functions
The mechanics of rock support is complex, and no
models exist that can fully explain the interaction of
various support components in a rock support system.
Nevertheless, Kaiser et al. (1996) summarized three
key support functions as: (1) reinforce the rock mass
to strengthen it and to control bulking, (2) retain
broken rock to prevent fractured block failure and
unraveling, and (3) hold fractured blocks and
securely tie back the retaining element(s) to stable
ground (Fig. 5).
Fig. 5 Three key functions (reinforce, retain, and hold) of rock
support (Kaiser et al., 1996).
The goal of reinforcing the rock mass using rock
bolts is not only to strengthen it, thus enabling the
rock mass to support itself (Hoek and Brown, 1980),
but also to control the bulking process, as rock bolts
prevent fractures from propagating and opening up.Fully grouted rebars, thread bars, or cable bolts are
well-suited for rock reinforcement.
Under high stress conditions, fractured rocks
between the reinforcing or holding elements may
unravel if they are not properly retained. Widely used
retaining elements are wire mesh, reinforced
shotcrete, strap, steel arch, or cast-in-place concrete.
Shotcrete needs to be reinforced by fiber or mesh to
increase its tensile strength and toughness.
Mesh-reinforced shotcrete or mesh over shotcrete
offers a much superior retaining function underrockburst conditions. In conventional rock support
systems, the retaining element is often the weakest
ReinforceRock mass
bulking
Hold
Retain
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link. A chain is only as strong as its weakest link. So,
if we want to increase the overall capacity of the rock
support system, the problem of weak retaining
elements and its connection to the reinforcing or
holding elements must be addressed.
When brittle rock fails, it is always associated with
large rock mass bulking. When a seismic event
occurs, this rock may also be subjected to large
impact energy, and when failing in an unstable
manner, stored strain energy may be released,
leading to rock ejection. Therefore, the installed rock
support system must be able to absorb dynamic
energy while also accommodating large sudden rock
deformations due to rock failure with associated
bulking. The holding function is needed to tie
retaining elements of the support system and the
loose rock back to stable ground, to dissipate
dynamic energy due to rock ejection and rock
movement, and to prevent gravity-driven falls of
ground. When rockburst damage is anticipated,
yielding holding elements such as conebolts and high
capacity friction bolts must be used in the support
system. The retaining component in a yielding
support system must also be able to tolerate large
tunnel convergence without self-destruction while
at the same time absorbing large dynamic energy. A
yielding rock support system is a system in harmony
with its surrounding, failing rock mass. As a
consequence, heavy continuous shotcrete rings are
often too stiff as they cannot deform with the bulking
rock.
The three support elements providing the
reinforcement, retaining, and holding functions do
not act independently. Therefore, these rock support
elements have to be well connected, forming an
integrated rock support system. The connection
between the retaining elements and the reinforcing
and holding elements deserves special attention to
ensure optimal overall capacity of the support system.Fig. 3 illustrates that all three support functions are
needed in an effective rockburst support system no
matter what the rockburst damage mechanism or
damage severity is.
3.2 Rockburst support design principles
In underground construction, strategy is the art of
commanding the entire mining or tunneling operation.
Tactic, on the other hand, is the skill of using various
tools for the construction and for dealing with
immediate needs in the field. Most engineers areforced to be tacticians as everyday tasks make them
think of how to deal with the most immediate
problems. To think strategically is more difficult and
often demands long-term thinking to get out of the
reactive mode to rockburst damage.
As Ralph Waldo Emerson, an American essayist,
philosopher and poet (18031882), said, As to
methods there may be a million and then some, but
principles are few. The man who grasps principles
can successfully select his own methods. The man,
who tries methods, ignoring principles, is sure to
have trouble. Realizing the importance of
understanding rockburst support design guiding
principles, Cai and Champaigne (2009) summarized
field experiences into a few simple and
easy-to-understand principles (Fig. 6).
Fig. 6 Summary of seven rockburst support design principles.
The first principle is to avoid rockburst whenever
possible. The supreme excellence in rock support in
burst-prone ground is to avoid rockburst conditions.
Hence, the best strategy is to stabilize the rock
without fighting against the loads and stresses in the
rocks using heavy rock support. Methods to avoid
rockburst risks include changing tunnel location, use
of different excavation shapes, changing the stope
size and/or shape, altering mining sequence and
potentially switching mining methods.
The second principle advocates the use of yielding
support in bursting grounds. When a brittle rock fails,
it is always associated with large rock dilation and
may be subjected to large impact energy. Therefore,
the installed rock support system must be deformable
and able to absorb dynamic energy. It is often
un-economical to prevent rockburst damage from
happening by increasing the load capacity of rock
support. The support behavior must be fundamentally
changed to a deformable yielding system that is able
to tolerate large tunnel convergence without
self-destruction while absorbing dynamic energy,thus providing support to ensure safety and
serviceability of the tunnel. A yielding rock support
7. Anticipateand be
adaptable
6. Cost-
effectiveness
5. Simplicity
4. Use
integrated
system
3. Address
the weakest
link
2. Useyielding
support
1. Avoid
rockburst
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system is a system in harmony with its surrounding
failing rock mass.
A chain is only as strong as its weakest link. In
conventional rock support systems, the retaining
element is often the weakest link and connection
between bolts and screen often fails in large
rockburst events. Consequently, the effectiveness of
a rock support system comprised of rock bolts and
mesh depends on their capacity, but most importantly
on the strength and capacity of the connections
between the bolts and the mesh. Unfortunately,
design procedures for rock support design focus
mostly on checking how much load a rock bolt can
carry, or how much energy the rock bolt can dissipate.
The failure of the rock mass between the bolts and
the impact of this failure on the rock support system
is often not considered in design. The selection of
surface support elements and the strength of the
connections must be matched with the capacity of the
bolts.
As a fundamental requirement, holding elements
need to be combined with reinforcing elements such
as rebars and surface support elements such as mesh
and shotcrete to form a rock support system. There is
no such thing as a super bolt or super liner that
can be used alone to combat rockburst problems.
Quite often, we need a rock support system that is
comprised of different rock support components,
because as indicated in Fig. 3 all three support
functions (reinforce, retain, and hold) are needed to
form an effective rock support system. Some support
components have multiple roles but may be strong in
one aspect and weak in another. It is essential that
various support elements be combined to form an
integrated support system. This is the principle of
using an integrated system.
The fifth principle is the simplicity principle.
Simplicity is powerful. Rock support elements
should be relatively easy to be manufactured,
installed, and maintained. Regardless of how
effective it is, if a rock support element is
complicated to manufacture and the cost is high,
operators will be reluctant to use it. If it is difficult to
install and production is adversely affected, its
acceptance by the mine operators and the miner will
suffer. When it comes to rock support in burst-prone
ground, it is always beneficial to follow Albert
Einsteins adviceMake everything as simple as
possible, but not simpler.
Unfortunately, there is still a wide-spreadassumption that rockburst-resistant support is
expensive for use in highly stressed ground. While
mining companies aim at reducing cost in order to
stay competitive, they cannot do so at the expense of
safety. The consequence of rockburst can be extreme,
ranging from damage to underground opening with
high rehabilitation costs, damage to mining
equipment, loss of production, permanent loss of
parts of ore bodies, to injury and fatalities. The cost
associated with these items can be extremely high.
For example, it is estimated that the rehabilitation
cost may be 10 to 20 times higher than the initial
development cost in underground hard rock mines. A
major rockburst may shut down mine production or
tunneling operations for an extended period of time.
In other words, if the price tag for rockburst damage
is high, the cost of preventing it in the first place,
using a rockburst resistant rock support system, can
be remarkably low. Damage prevention and controlin burst-prone ground is most cost-effective.
The last principle advocates the ability to
anticipate and to adapt. Burst-prone ground
conditions and rockburst damage severity potential
change constantly, and it is unrealistic to have a fixed
design that cannot be changed. The underground
excavation and rock support method therefore must
be responsive to a variety of ground conditions that
can be encountered. The art of rock support in
burst-prone ground is not to rely on the low
likelihood of unexpected ground behaviors, but onthe readiness to manage them with an effective rock
support system that is unbeatable.
By understanding the seven principles (Fig. 6), the
ability to safeguard workers and investment risk can
be improved. These core principles must guide rock
support design.
3.3 Rockburst support acceptability criteria
Rock support in burst-prone ground differs from
conventional rock support where controlling
gravity-induced rockfalls and managing shallow
zones of loose rock are the main concern. In additionto these design issues, rock support in burst-prone
ground needs to resist dynamic loading and large
rock bulking due to violent rock failure.
The classical approach in engineering design
assesses the safety margin by the ratio between the
capacity (strength or resisting force) of support
elements and the demand (stress or disturbing force).
Rock support design for burst-prone ground can
follow the same approach but the capacities must
also be assessed in terms of load, displacement, and
energy dissipation capacities. First, the expectedloading condition or demand on the support is
determined; next, various support elements are
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dimensioned and then integrated into a support
system to achieve a support capacity that exceeds the
anticipated demand. The demand is influenced by
many factors, such as opening size and shape, rock
mass properties, in-situ and mining-induced stress
level and orientation, seismic source type and
characteristics, stress wave magnification, support
conditions and properties, etc. In burst-prone ground,
the following four design acceptance criteria need to
be simultaneously assessed. Not all of them may be
critical and thus not all will, in a given case, affect
the final support system.
(1) Force criterion
The load factor of safety ( LoadFS ) is defined by
Load
Support load capacity
Load demandFS (1)
In general, the force criterion covers the design forboth static and dynamic loads. Under dynamic
loading conditions, the dynamic acceleration will
increase the load demand and movement may be
triggered. If this is the case, a deformable support
system has to be used to dissipate some of the energy
demand until the static demand drops below the
support load capacity.
(2) Displacement criterion
Even if an effective rock support system is
installed, rock fracturing cannot be prevented if the
stress exceeds the rock mass strength. When a rockfractures, as its volume increases, it bulks. Volume
increase in the tangential loading direction is
restrained and the fractured rocks can only deform in
the radial direction into the excavation, leading to
large bulking deformations near the wall. Hence, the
installed rock support system must have sufficient
displacement capacity to meet or exceed the
displacement demand. The displacement factor of
safety (Disp
FS ) is defined by
Disp
Support displacement capacity
Displacement demandFS (2)
(3) Energy criterion
When a rock block is ejected from the excavation
boundary, it possesses kinetic energy, or in the case
when a rockfall is triggered, the energy demand is
increased by the change in potential energy. Hence,
the designed energy absorption capacity of the
support system must meet or exceed the energy
demand. The energy factor of safety ( EnergyFS ) is
defined by
Energy
Support energy capacity
Energy demandFS (3)
When a rock with mass m is ejected from the
tunnel roof at an ejection velocity ve, the support
system with a large displacement capacity contains
the ejected rock after a displacement of ds, the energy
demand (Kaiser et al., 1996) is
2
e s
1
2E mv mgd (4)
where gis the gravitational acceleration. Hence, the
support system for rock failing in the roof must be
able to absorb this amount of kinetic energy.
(4) System compatibility criterion
The previous three design criteria, i.e. load,
displacement, and energy criteria, are intended for
the design of reinforcement and support holding
elements. However, these elements can only work to
achieve their design capacity if the surface support
elements are strong and can effectively transfer the
loads to the reinforcement and holding elements.
There is a strong interaction between the
reinforcement/holding elements and the surface
support elements, i.e. the capacity of the
reinforcement/holding elements depends on the
capacity of the surface support elements, and the
capacity of the surface support elements also depends
on the capacity, as well as the spacing of the
reinforcement/holding elements.
An optimal rock support system is one with
compatible and balanced support elements where allsupport elements work in harmony to contribute their
capacities to the fullest. The holding and the surface
retaining elements capacity of the system must be
compatible with rock load and rock deformation, and
holding elements capacity must be compatible with
the surface retaining elements capacity. In design, it
is difficult to calculate the demand for surface
support elements. Hence, empirical design methods
are often used but it is important to ensure that the
load, displacement, and energy capacities of surface
support are compatible to those of thereinforcement/holding elements.
4 Rockburst support design using
BurstSupport
4.1 Design procedure
As explained above, rockburst support design is to
meet the load, displacement, and energy demands
with appropriate support capacities, under given
ground and excavation conditions.Geological and geotechnical data are the
foundation for all mine and tunnel design. Because
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rock mass behavior can vary drastically in a mine or
along a tunnel, it is necessary to establish rock mass
domains according to varying geological,
geometrical, and seismic data considerations. First,
the rock mass along a tunnel alignment is typically
divided into domains based on seismic activities,which is mostly influenced by mining activities. Next,
within each domain, sub-zones are identified within
which the key engineering design parameters, such as
in-situ or mining-induced stress, lithology, and rock
mass quality (intact rock strength, discontinuity
frequency, etc.), are comparable.
In each design domain, one needs to estimate the
anticipated seismic event magnitude and event
location as well as potential rockburst damage
mechanisms, and calculate the load, displacement,
and energy demands on the rock support for the
dominant rockburst damage mechanism. It is often
difficult to know in advance which type of rockburst
damage mechanism is likely to occur and dominate
the design as the expected damage severity controls
the demand. Hence, all three rockburst damage
mechanisms need to be analyzed separately before
the critical support demand can be identified. Then,
the best decision on rock support system selection
can be made in view of the worst-case scenario (the
controlling criterion). Furthermore, it can be assessed
whether rock support should be designed to prevent
the initiation of damage or whether the rock support
system must be designed to control the failure
process with related deformations and energy release.
Next, one will have to examine all available rock
support elements and pick the best combination of
support elements to form an integrated rock support
system with the desired support capacities exceeding
the anticipated load, displacement, and energy
demands previously determined. In recent years,
many new support products (Ortlepp and Erasmus,2005; Varden et al., 2008; Potvin, 2009; Bucher et al.,
2010; Cai et al., 2010; Doucet and Gradnik, 2010; Li
and Charette, 2010; Cai and Champaigne, 2012) have
been developed. This provides an enhanced pallet of
support options for the users but also introduces a
level of uncertainty as not all new products act in the
same manner and have a proven track record.
Prudence is advised when considering products as
specified performances may not be achievable under
field conditions.
Support systems for rockburst conditions areselected on the basis of their load-displacement
characteristics and the expected nature and severity
of rock mass failure, by combining different holding,
reinforcing, and retaining elements and ensuring the
overall integrity of the support system. This is
achieved by considering compatible support elements
to form an integrated rock support system, thereby
eliminating the weakest link in the system. A
satisfactory design can rarely be achieved in one step,
demanding various iterations and comparisons of
design options.
4.2 Design tool
Mine geology and infrastructures are complex and
three-dimensional in nature. Presently in mining
practice, either rockburst support is selected based on
site specific or global experience or the design is
performed using often simplistic spreadsheet
calculations. However, rock support design cannot be
carried out in a systematic manner without taking
into account geometric (mine excavations) and
geological complexities. Furthermore, when
performing such time and effort consuming designs
manually, costly mistakes may be made if attention is
not paid to the interaction of the various influence
factors outlined above.
A design tool called BurstSupport is being
developed at Laurentian University, Canada, with
support from CEMI (Centre for Excellence in Mining
Innovation), NSERC (The Natural Sciences and
Engineering Research Council of Canada), andseveral mining companies (see acknowledgements)
to address the needs of industry. This tool
encapsulates some of the research findings from the
Canadian Rockburst Support Handbook (Kaiser et al.,
1996) and integrates many recent research outcomes
from other investigators. As well, it facilitates the
interactive and iterative process of rockburst support
design. Parties potentially interested in participating
in the further development of the tool are invited to
contact the authors to discuss project sponsorship.
BurstSupport is a standalone Windows-basedsoftware tool which enables the user to assess load,
displacement, and energy demands at multiple drift
locations by simultaneously considering anticipated
seismic event magnitude and location, in-situ and
mining-induced stress conditions, drift orientation,
and rock mass quality. Rock support can be selected
from a pre-defined support database and assigned to
drifts at various locations. Furthermore, 3D mine
structures and geological structures can be imported
into the tool for easy manipulation (rotation, zoom,
pan, etc.). The screenshot presented in Fig. 7 showsthe main user interface for effective display 3D
geometrical objects for data fusion and integration.
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224 Peter K. Kaiser et al. / J Rock Mech Geotech Eng. 2012, 4 (3): 215227
Fig. 7Main user interface of BurstSupport.
As shown in Fig. 8, the user can specify a design
seismic event or multiple seismic events (shown as
balls in the figure) which may occur in the mine
during operation and calculate resultant peak particle
velocities (ppv) along the drifts that requirerockburst support consideration. The calculation of
ppvis based on the scaling law given by Kaiser et al.
(1996) as
Fig. 8Calculatedppvalong the drifts (ppv=0.0890.36 m/s).
0
aMppv C
R
(5)
where M0 is the seismic moment (GNm), R is the
distance between the drift location and the seismic
source (m), and a
*
and C
*
are empirical constants thatshould be calibrated for each site. Seismic moment
can be related to the event magnitude. Based on the
analysis of seismic data from a global database, it
was found that a* in Eq. (5) can be fixed at a
*=0.5
and C*values are determined from lg(Rppv)-lg(M0)
plots with a reasonable upper-bound limit (e.g. at
95% confidence level), where is the stress drop.
The values of ppv shown in Fig. 8 are calculated
using the scaling law with two sequential seismic
events whose Richter magnitudes are 3.0 (Event #0)
and 2.0 (Event #1), respectively. The maximumvalue of ppv due either event at a drift location is
shown in Fig. 8.
Alternatively, the BurstSupport tool allows direct
import of ground motion parameters to drift locations,
calculated using the synthetic ground motion (SGM)
approach. The SGM technique was widely used in
earthquake study (Boore, 2003) and has attracted
some attention in mining (Hildyard, 2001; Hildyardand Milev, 2001). The SGM approach generates the
modeled near-field waveforms by considering
fault-slip mechanism, stress drop, slip direction, slip
time, and slip amount. The source waves are then
propagated in the media by a nonlinear site response
analysis using 3D analytical or numerical models
which can effectively consider the influence of
excavations, geological structures, and mining-
induced stress changes on wave propagation. More
representative ppv and ppa values at the drift
locations can be obtained from SGM simulations.
In-situ and mining-induced stresses influence the
depth of failure and hence the required amount of
rock support. Stress analysis can be performed using
an external 3D FEM, FDM, or BEM tools, and stress
component values on each node along the drift
centrelines can be imported into the BurstSupport
tool. The maximum tangential stress in a plane
perpendicular to the drift axis is found and the depth
of failure is estimated using the empirical method
described by Kaiser et al. (1996) and Martin et al.
(1999). An example of calculated depth of failure is
presented in Fig. 9. When calculating the anticipated
depth of failure (df), the tool takes the rock mass
strength, drift orientation, and stress magnitudes into
account. By comparison of Figs. 8 and 9, it can be
seen that the greater depth of failure in this case is
not dominated by the ground motion as deeper
damage is predicted at locations of lower ppv. In
addition to ppv, other factors such as stress
orientation and rock strength affect the depth of
failure at this location.
Fig. 9 Calculated depth of failure along the drifts (df =0.5
1.2 m).
Event #0
Event #1
Event #0
Event #1
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A convenient feature of the tool is that the user is
able to manually select rock support systems with
defined support capacities and assign/visualize the
rock support pattern to a specific section of the drift.
Suggested values of load, displacement, and energy
capacities of most commercially available rock bolts
are included in the database but the user can also
modify or define support properties (Fig. 10).
Through an interactive and iterative process of
adjusting rock support type and bolt spacing, the
factors of safety for load, displacement, and energy
can be checked to meet the minimum requirements.
One example of calculated factor of safety for the
displacement demand is presented in Fig. 11. The
result shows that the lowest factor of safety for the
displacement demand is 2.65, because high
displacement yielding rock bolts (with adisplacement capacity of 300 mm) are applied. If
rock bolts with a 50 mm displacement capacity were
used, the factor of safety would be less than one in
some drift sections (not shown). On the other hand, if
yielding bolts with 150 mm displacement capacity
are used, the minimum factor of safety for the
displacement demand is 1.3. For the drifts under
consideration, a decision can therefore be made to
select rock bolts with a displacement capacity of 150
mm to optimize the design. In this fashion, the
rockburst damage problem can be addressed
proactively by prescribing cost-effective rock support
systems to the mine drifts. Caution should though be
exercised as the cumulative effect of various input
parameters may lead to large variability in support
demand. Sensitivity analyses or parametric studies
are advised before a final support system is selected
(see below).
Fig. 10Screenshot of defining rock support window.
Fig. 11 Factor of safety for displacement demand visualized
on mine drifts (FS= 2.657.51).
As illustrated above, an optimal support design
strategy is obtained following an iterative process
wherein the tool effectively assists in achieving
optimization and verification tasks. Another useful
feature of the tool is the statistical analysis of
prescribed rock support (by providing statistics of
some parameters) for the drifts such as the minimum
factors of safety and the total numbers of rock
support in one particular section of the drift so as to
facilitate mine planning. For example, the total
numbers of rock bolts in one mine level can be found
easily from the statistical analysis. The total numbers
of rock bolts thus calculated consider the bolt pattern
and the 3D geometry of the drifts.4.3 Design verification
Although some model-based design and numerical
methods are used, rock support system design for
underground excavations is largely dependent on
empirical methods and practical experience.
Whatever design method is applied, a final design is
best arrived at based on an observation design
method. This is particularly the case for bursting
conditions because of large uncertainties associated
with the seismic event magnitude and location, therock mass strength, local stress, and rock support
capacities.
The observational design approach, advocated by
Peck (1969), is highly recommended for use in
rockburst support design. The fundamental principles
of the observational design approach include
avoiding difficult ground conditions, letting the rock
support itself (Hoek and Brown, 1980), conducting
robust design, having an adequate field monitoring
plan, having plans for contingency measures, and
adjusting construction methods according to exposed
condition. Observational methods utilize monitoring
Event #0
Event #1
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226 Peter K. Kaiser et al. / J Rock Mech Geotech Eng. 2012, 4 (3): 215227
as an integral part in the rock support system design
process. The underlying logic is that a design is not
complete until the design assumptions have been
verified and the structures performance has been
matched with performance predictions.
Field monitoring provides input for feedback loopsin the design process. Analysis of microseismic
monitoring may indicate that the design seismic
magnitude and location needs adjustment; analysis of
convergence data and depth of failure data may
suggest that the adopted rock mass properties, or
even the in-situ stress field, needs modification;
observation of rock support system performance may
show that the selected support system or support
component connects need modification. The
BurstSupport tool can be used by ground control
engineers to conduct this design verification. A
rational design combined with field observation and
monitoring is the key to the success of rockburst
support design in burst-prone ground.
5 Conclusions
Rockbursting is a complex mining-induced
phenomenon occurring in deep underground
construction. Much effort has been put into researchto understand why rockburst happens and what the
anticipated damage processes are. Unfortunately, due
to the complexity of rock mass and the boundary
conditions, we still do not have great confidence in
predictive means and reality repeatedly reminds us of
current deficiencies. As mining progresses to greater
depths, violent rock failure cannot be avoided and it
will have to be dealt with on a routine basis by
implementing rockburst resistant support strategies.
The first step in mastering the science and art of
rockburst support design is to understand rockburst
mechanisms and identify the main factors that
influence rockburst damage. Next, it is imperative to
understand the seven principles of rockburst support
design and three important functions of rock support,
i.e. reinforce, retain, and hold. Most importantly, four
design acceptability criteria, i.e. load, displacement,
energy, and system compatibility criteria, must be
satisfied by any design. By following these design
acceptability criteria, a clear distinction between the
rockburst support design and conventional rock
support design is made.
Finally, realizing that the design procedure for
rock support design in burst-prone ground is iterative,
a design tool called BurstSupport is being developed
to assist ground control engineers to quickly and
systematically evaluate different rockburst support
options in a user-friendly manner. The BurstSupportdesign tool, which considers seismic event and
ground motion, as well as rock mass quality and
mining-induced stresses, assesses the load,
displacement, and energy demands, and provides
ground control engineers with a new set of tools for
mine planning and geomechanics design. It is
envisioned that rockburst risk management can be
significantly improved using this tool.
Acknowledgements
Financial supports from CEMI, LKAB,
MIRARCO, NSERC, VALE, and XSTRATA
NICKEL are deeply appreciated. Software
development effort from Henry Li is greatly
appreciated. Technical advice and direction from
Damien Duff of CEMI, Denis Thibodeau of VALE,
Lars Malmgren of LKAB, and Brad Simser of
XSTRATA NICKEL are also thankfully
acknowledged.
References
Blake W, Hedley D G F. Rockbursts, case studies from North American
hardrock mines. Englewood: Society for Mining, Metallurgy, and
Exploration, 2003.
Boore D M. Simulation of ground motion using the stochastic method.
Pure and Applied Geophysics, 2003, 160 (3): 635676.
Bucher R, Roth A, Roduner A, Temino J. Ground support in high stress
mining with high-tensile chain-link mesh with high static and
dynamic load capacity. In: Proceedings of the 5th International
Seminar on Deep and High Stress Mining. Santiago: [s.n.], 2010:
273282.
Cai M, Kaiser P K, Tasaka Y, Maejima T, Morioka H, Minami M.
Generalized crack initiation and crack damage stress thresholds of
brittle rock masses near underground excavations. International
Journal of Rock Mechanics and Mining Sciences, 2004, 41 (5):
833847.
Cai M, Champaigne D. The art of rock support in burst-prone ground.
Keynote Lecture. In: RaSiM 7: Controlling Seismic Hazard and
Sustainable Development of Deep Mines. [S.l.]: Rinton Press, 2009:
3346.
Cai M, Champaigne D, Kaiser P K. Development of a fully debonded
conebolt for rockburst support. In: Proceedings of the 5th
8/10/2019 Design of rock support system under rockburst condition.pdf
13/13
Peter K. Kaiser et al. / J Rock Mech Geotech Eng. 2012, 4 (3): 215227 227
International Seminar on Deep and High Stress Mining. Santiago:
[s.n.], 2010: 329342.
Cai M, Champaigne D. Influence of bolt-grout bonding on MCB conebolt
performance. Int. J. Rock Mech. Min. Sci., 2012, 49 (1): 165175.
Doucet C, Gradnik R. Recent developments with the RoofexTM bolt. In:
Proceedings of the 5th International Seminar on Deep and High
Stress Mining. Santiago: [s.n.], 2010: 353366.
Durrheim J, Roberts M K C, Haile A T, Hagan T O, Jager A J, Handley M
F, Spottiswoode S M, Ortlepp W D. Factors influencing the severity
of rockburst damage in South African gold mines. J. South Afr. Inst.
Min. Metall. 1998: 5357.
Heal D, Potvin Y, Hudyma M. Evaluating rockburst damage potential in
underground mining. In: Proceedings of the 41st U.S. Symposium on
Rock Mechanics (USRMS). Golden: American Rock Mechanics
Association, 2006.
Hedley D G F. Rockburst handbook for Ontario hardrock mines.
CANMET SP92-1E, 1992.
Hildyard M. Wave interaction with underground openings in fractured
rock. Ph.D. Thesis. Liverpool: University of Liverpool, 2001.
Hildyard M W, Milev A M. Simulated rockburst experiment: development
of a numerical model for seismic wave propagation from the blast,
and forward analysis. J. South Afr. Inst. Min. Metall., 2001: 235245.
Hoek E, Brown E T. Underground excavations in rock. London: Institution
of Mining and Metallurgy, 1980.
Kaiser P K, Diederichs M S, Martin C D, Sharp J, Steiner W.
Underground works in hard rock tunnelling and mining. Keynote at
GeoEng2000. Melbourne: Technomic Publishing Co., 2000:
841926.
Kaiser P K, Tannant D D, McCreath D R. Canadian rockburst support
handbook. Sudbury, Ontario: Geomechanics Research Centre,
Laurentian University, 1996.
Li C, Charette F. Dynamic performance of the D-bolt. In: Proceedings of
the 5th International Seminar on Deep and High Stress Mining.
Santiago: [s.n.], 2010: 321328.
Martin C D, Kaiser P K, McCreath D R. Hoek-Brown parameters for
predicting the depth of brittle failure around tunnels. Can. Geot. J.,
1999, 36 (1): 136151.
Ortlepp W D, Erasmus P N. Dynamic testing of a yielding cable anchor. In:
Proceedings of the 3rd Southern African Rock Engineering
Symposium. South Africa: [s.n.], 2005.
Ortlepp W D, Stacey T R. Rockburst mechanisms in tunnels and shafts.
Tunnelling and Underground Space Technology, 1994, 9 (1): 5965.
Ortlepp W D. Rock fracture and rockbursts an illustrative study.
Johannesburg: The South African Institute of Mining and Metallurgy,
1997.
Ortlepp W D. Observation of mining-induced faults in an intact rock mass
at depth. International Journal of Rock Mechanics and Mining
Sciences, 2000, 37 (1/2): 423436.
Peck R B. Advantages and limitations of the observational method in
applied soil mechanics. Geotechnique, 1969, 19 (2): 171187.
Potvin Y. Surface support in extreme ground conditionsHEA Mesh. In:
Safe and Rapid Development in Mining. Perth, Western Australia:
[s.n.], 2009: 111119.
Stacey T R. Support of excavations subjected to dynamic (rockburst)
loading. In: Proceedings of the 12th ISRM Congress. [S.l.]: Taylor
and Francis Group, 2011: 137145.
Varden R, Lachenicht R, Player J, Thompson A, Villaescusa E.
Development and implementation of the Garford dynamic bolt at the
Kanowna Belle mine. In: Proceedings of the 10th Underground
Operators Conference. Launceston, Tasmania: [s.n.], 2008: 95102.